如何在左联接中获取另一个表的计数

Question

我有多张表

post
    id  Name
    1   post-name1
    2   post-name2

user
    id  username
    1   user1
    2   user2

post_user
    post_id   user_id
    1         1
    2         1

post_comments
    post_id   comment_id
    1         1
    1         2
    1         3

我正在使用这样的查询：

SELECT post.id, post.title, user.id AS uid, username
FROM `post`
LEFT JOIN post_user ON post.id = post_user.post_id
LEFT JOIN user ON user.id = post_user.user_id
ORDER BY post_date DESC

它按预期工作。但是，我也希望获得每个帖子的评论数。因此，如何修改此查询，以便获得评论数。

有任何想法吗？

Answer 1

SELECT post.id, post.title, user.id AS uid, username, COALESCE(x.cnt,0) AS comment_count
FROM post
LEFT JOIN post_user ON post.id = post_user.post_id
LEFT JOIN user ON user.id = post_user.user_id
LEFT OUTER JOIN (SELECT post_id, count(*) cnt FROM post_comments GROUP BY post_id) x ON post.id = x.post_id
ORDER BY post_date DESC

编辑：在没有任何评论的情况下使其成为外部联接

EDIT2：更改IsNull为Coalesce